1. Field of the Invention
This invention relates to a thin-film transistor that uses a semiconductor of amorphous silicon.
2. Description of the Prior Art
In recent years, there has been a good potential market for active-matrix display devices, as large-scale display devices that use liquid crystals, etc., in which thin-film transistors (TFT) made with the use of a semiconductor of amorphous silicon (a-Si) are formed in a matrix on an insulating substrate such as glass, etc.
FIGS. 6(a) to 6(d) show a process for the production of a conventional TFT. First, a gate electrode 22 is formed on an insulating substrate 21 made of glass, etc. Then, a first insulating film 23, a non-doped a-Si semiconductor film 24, and a second insulating film 25 are disposed thereon, in that order (FIG. 6(a)). Then, the second insulating film is patterned so as to remain on the gate electrode 22 (FIG. 6(b)), followed by the disposition of a phosphorus-doped n.sup.+ -a-Si film 26, and the a-Si semiconductor film 24 is then patterned (FIG. 6(c)). Then, a metal film such as Al, Ti, Mo, etc., is deposited on the entire surface of the substrate, followed by the patterning of this metal film to form a source electrode 27 and a drain electrode 28. Thereafter, a picture-element electrode 29 is formed from a transparent conductive film so that one part of the drain electrode 28 is overlapped, resulting in a TFT, the flat surface of which is shown in FIG. 7.
The conventional liquid-crystal display devices are disadvantageous in that (1) it is difficult to obtain good R.sub.off characteristics, (2) short-circuits occur readily between the gate electrode and the source electrode, (3) scattering of the picture-displaying characteristics occurs readily, (4) the bus bar of the source electrode breaks readily, (5) short-circuits between the source electrode and the picture electrode occur readily, and (6) the ratio of the surface area of the picture-element electrode to the surface area of the liquid-crystal display panel is low.
First, it is explained as follows why it is difficult to obtain good R.sub.off characteristics. For example, when the width L of the second insulating film 25 of the TFT is 10 .mu.m, when the width W of the n.sup.+ -a-Si film 26 is 30 .mu.m, and when gate voltage is not applied, there is scattering on the order of 10.sup.4 -10.sup.11 .OMEGA. of the resistance inside and outside of the panel between the source and the drain, so that a satisfactory display cannot be obtained when the liquid-crystal cell is assembled with the said TFT.
The reason is that at the time of the disposition of a metal film constituting the source electrode 27 and the drain electrode 28, the edge part of the a-Si semiconductor film 24 (i.e., the stippled area shown in FIGS. 7 and 8) and the metal film for the source electrode 27 and the drain electrode 28 react, resulting in a conductive reaction layer.
Next, it is discussed why short-circuits occur readily between the gate electrode and the source electrode. The TFT at each point of intersection in the active-matrix substrate that is formed by the disposition of a plurality of TFTs in a matrix on an insulating substrate is driven in a line-sequential mode. That is, a scanning signal is input from one gate bus bar to be scanned, and data signals are input from each source bus bar. There are many points of intersection between the gate bus bars and the source bus bars. For example, in a matrix of 250.times.250, there are 62,500 points of intersection. If, among these many points of intersection, even one permits a leak between the gate and source, a cross-shaped line defect inevitably occurs between the corresponding gate bus bar and the corresponding source bus bar, so that a satisfactory display cannot be obtained and the yield of the active-matrix substrate becomes zero. More certainty of the insulation between the gate and the source is required with an increase in the number of gate bus bars and source bus bars.
Next, it is explained why scattering of the picture-displaying characteristics occurs readily.
With a conventional active-matrix substrate that uses the above-mentioned TFTs, none of the gate bus bars and none of the source bus bars are equipotential, so the following problems occur. The source bus bars are not equipotential to each other. Accordingly, a difference in the threshold voltage of these TFTs arises because of static electricity created between the TFTs connected to different source bus bars during the manufacturing process, and when liquid-crystal cells are incorporated into this active-matrix substrate and used as a display device, stripes appear along the source bus bars. Also, with each gate bus bar, the same trouble as with the source bus bars occurs, and a satisfactory picture-display cannot be obtained.
Next, the reason for the source bus bars breaking readily is discussed. One method often used to prevent the above-mentioned defect in which short-circuits between the gate and the source readily occur is to introduce an a-Si semiconductor film, an insulating film, etc., at the points of intersection between the gate bus bars and the source bus bars. In the conventional method shown in FIGS. 7 and 9, an a-Si semiconductor film 24 and a protective insulating film 25 have been introduced at the points of intersection between the gate bus bars 22 and the source bus bars 27.
However, when this method is used, it is not uncommon for breaking of the portions of the source bus bars 27 corresponding to the step-portions of the protective insulating film 25, the a-Si film 24, and the a-Si film to occur (shown in FIG. 9 by the arrows).
Next, the reason for short-circuits occurring readily between the source electrode and the picture-element electrode is discussed.
In the conventional TFT shown in FIGS. 6 and 7, the source electrode 27 and the picture-element electrode 29 are on the same insulating film, so that in the stippled area of FIG. 7, etching of the source electrode 27 and the picture-element electrode 29 is not satisfactory, and leaks occur readily.
Next, the reason why the ratio of the surface area of the picture-element electrode to the surface area of the liquid-crystal display panel is low is discussed below. In order to prevent short-circuits between the source electrode and the picture-element electrode, the source electrode and the picture-element electrode must be separated on the insulating film. If the gate electrode 22 and the picture-element electrode 29 are overlapped in the same plane, a parasitic capacity between the gate electrode and the picture-element electrode is created, which has a bad effect on large-capacity displays. For this reason, so that the gate electrode 22 and the picture-element electrode 29 cannot be overlapped in the same plane, the gate electrode 22 and the picture-element electrode 29 must be separated in a plane by the stippled area shown in FIG. 7. As mentioned above, when a display is made with an active-matrix that uses the conventional TFT, a non-lighting region is required in the stippled area shown in FIG. 7, which lowers the ratio of the surface area of the picture-element electrode to the surface area of the liquid-crystal display panel.